Combustion is a major source of pollutants. One class of pollutants, oxides of nitrogen, or NO.sub.x, (NO or nitric oxide, NO.sub.2 or nitrogen dioxide, and N.sub.2 O or nitrous oxide) contribute to acid rain, smog, global warming, and ozone depletion. As NO.sub.x emissions from combustion sources primarily consist of nitric oxide, an understanding of how NO is generated is important. There are three principal mechanisms which produce NO during combustion: 1) the fuel NO mechanism; 2) prompt NO mechanisms; and 3) the Zeldovich mechanism, also known as "thermal NO". As "fuel NO" is produced via oxidation of nitrogen contained with the fuel, it is not generally an issue for combustion sources using gaseous fuels as these contain little, if any nitrogen compounds. The majority of "prompt NO" is formed through three paths with the significance of each path dependent upon a multiplicity of variables such as pressure, fuel/air ratio, temperature, and concentrations of other compounds. "Thermal NO" is formed by the oxidation of atmospheric nitrogen, N.sub.2, and increases exponentially with combustion temperature. In most combustion applications, the majority of NO, and thus NO.sub.x in general, is produced by the "thermal NO" mechanism, with "prompt" and other mechanisms playing a minor role. As such, combustion processes which decrease the combustion temperature, and thus greatly reduce the production of thermal NO, can have a large effect on the entire production of NO.sub.x. Further information on this subject may be found in Bowman, C. "Control of Combustion-Generated Nitrogen Oxide Emissions: Technology Driven By Regulation", Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute 1992, pp. 859-878, which is incorporated by reference herein.
The best strategy to control pollution is to minimize its formation at the combustion source. Current research and development efforts are mainly dedicated to lowering emissions through re-engineering of conventional non-premixed combustion systems. Non-premixed combustion is the scientific term given to the combustion process where fuel and oxidizer (usually air) mix and burn concurrently. Non-premixed flames are not clean. Unmitigated, they generally emit unacceptable levels of oxides of nitrogen (NO.sub.x) of over 200 parts-per-million (ppm), substantially higher than regulations allow for certain applications. The heating and power generation industries have recognized the need to develop premixed combustion systems as they are much cleaner than non-premixed combustion systems. In premixed combustion systems, as its name implies, gaseous fuel and oxidizer (usually air) are first mixed and then burned. It is desirable to separate the mixing and burning processes, as the user can then control the fuel-to-air ratio being delivered to the reaction zone. The major challenge facing research and development of premixed combustion systems is that the body of knowledge gained from conventional (i.e. non-premixed) burner development is not directly transferable to premixed systems due to differences in flame dynamics.
A burner requires a stable flame. For non-premixed burners, maintaining a stable flame in the zone where the fuel and air are mixing together is the important design criterion. On the other hand, the design of premixed burners have quite different requirements as the mixing occurs separately from the burning processes. Also, when burning the premixed fuel and air mixture (or "feed gas") the flame front propagates through the feed gas. In contrast, non-premixed flames do not propagate. The speed at which a premixed flame propagates through the mixture is called the flame speed. Flame speed is a function of the fuel/air equivalence ratio and turbulence intensities. A fundamental requirement of premixed burners is that the velocity of the premixed feed gas in the burner has to be greater than the flame speed. Otherwise, the flame will propagate upstream against the premixed feed gas stream and into the body of the burner. This condition is termed "flash-back," and must be avoided. To maintain a stable flame, an obstacle may be placed in the premixed feed gas to `anchor` the flame. The size of the flame anchor (also called a "flame-stabilizer" or "bluff-body") and their aerodynamic shapes can be optimized for given operating ranges and burner considerations. Although these anchors can help to prevent flash-back, a flame can become unstable and "blow-off" the anchor if the velocities are too high. This is particularly true for lean flames, as they tend to blow-off easily due to the excess air in the feed gas. The challenge for premixed burners which support lean flame conditions, is to have a design robust enough to eliminate both flash-back and blow-off occurrences.
Conventionally, stable premixed feed gas flames have been achieved by using a flame anchor, as described above. The obstruction generates a zone of zero axial flow on its upstream side and turbulent flow on its downstream side. As the fuel flows around the obstruction, it becomes turbulent and several regions of reverse flow are created where the fuel flow is actually circling back in a direction opposite to the original flow. This pattern, referred to as "recirculation," is relatively stable and prevents blowout when burner operating conditions are appropriately set.
Swirling flows have also been used to stabilize combustion in a variety of burners, both premixed and non-premixed. Swirl in these burners is generally created either by generating tangential flow motion in a cylindrical chamber, as in cyclone combustion chambers, or swirling a co-axial air flow. In both cases, the function of the swirl is to create a torroidal recirculation zone (TRZ). For non-premixed combustion, the role of the TRZ is to mix the fuel and air to allow for complete combustion, to stabilize the combustion process, to recirculate some fraction of the products, and to dictate the physical shape and length of the flame in these burners. In premixed burners, the TRZ created by the strong swirl creates a zone where the combustion zone is "anchored" due to an area of low flow velocities found within the TRZ.
Many attempts have been made to reduce NO.sub.x emissions from combustion sources in hopes of reducing the air pollution associated with the burning of hydrocarbon fuels. Pollution reduction methods generally fall into two categories. One category of reduction methods involve post-combustion remediation technologies, such as Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR), to reduce pollution after it has been generated in the combustion zone. Combustion modifications through burner design changes, such as burning lean, fuel-air staging, or flue gas recirculation, can reduce pollutant formation in the reaction zone. These methods constitute the second category of pollution reduction technologies. Taking into account tradeoffs with engineering considerations and other pollutants, low NO.sub.x burners generally burn with as much excess air (i.e. "lean") as possible, as this will reduce combustion temperatures and minimize thermal NO.sub.x production. Some NO.sub.x may still be produced by virtue of the prompt NO.sub.x mechanisms detailed earlier.
U.S. Pat. No. 4,021,188 to Yamagishi et al. ("the '188 patent") describes various non-premixed burner arrangements using both staged combustion and exhaust gas recirculation to decrease the production of NO.sub.x during combustion of hydrocarbon fuels and air. Staged combustion involves an initial combustion of a fuel rich-air mixture followed by a second combustion zone of the partially combusted fuel exhausted from the first reaction zone, generally in combination with a secondary air supply. The burner configurations disclosed in the '188 patent involve a fuel-rich, first combustion stage followed by a fuel-lean, low-temperature second combustion stage. Several of these configurations involve a swirling mechanism to mix the partially combusted gas discharged from the first combustion stage with a secondary air source. This swirling fuel-air mixture undergoes a second combustion stage at a lower combustion temperature than that of the first stage. The '188 patent discloses a number of apparatuses equipped with a swirling mechanism, such as blades or slits, to induce a rotating flow in the partially combusted gas discharged from the first combustion stage or in the secondary air supply.
In all of these apparatuses, the air component of the fuel-air mixture is supplied to the secondary combustion zone through a separate passage from a separate source from that of the partially combusted gas, which proceeds to the secondary combustion zone through a central passage that is either unobstructed or contains a central hub. The '188 patent also discloses that the partially combusted gas is cooled by a flow which develops as surrounding relatively low temperature combustion gas is attracted into the center of the rotating flow of the partial combustion gas, thereby inhibiting NO.sub.x formation from thermal NO.sub.x production. Thus, these apparatuses rely upon torroidal recirculation for flame stabilization.
For premixed combustion, another design has achieved a significant reduction in NO.sub.x production using single stage combustion without recirculation. U.S. patent application Ser. No. 08/033,878, filed Mar. 19, 1993 ("the '878 application), which is the parent of the present application, and has been previously incorporated by reference into the present application, discloses a lean premixed burner which generates a stable flame by swirling the edges of a thoroughly mixed fuel-air mixture without inducing recirculation. Unlike many conventional burners, including the second stage of the burners disclosed in the '188 patent, in which the fuel and air are mixed in the combustion zone, the '878 application discloses a premixed fuel-air mixture that is swirled gently by low swirl jets of air, or premixed fuel/air, introduced tangentially to the premixed stream of feed gas upstream of the reaction zone. The low swirl creates a divergent flow pattern that "anchors" the flame at the point where the flame speed balances the mass flow rate of the fuel-air mixture, thus stabilizing the flame zone without the use of recirculation.
A common parameter for characterizing the swirl intensity of swirl burners is in use throughout the combustion industry. The non-dimensional swirl number, S, is defined as the ratio of axial flux of angular momentum to the axial flux of linear momentum divided by the nozzle radius: ##EQU1## where R is the nozzle radius, and U and W are the mean axial and tangential components of the flow velocity within the swirl generator, respectively.
For a tangential air injection swirler, this integral form is commonly approximated by the following geometric form: ##EQU2## where r.sub..theta. is the radius of the swirl jets, A.sub..theta. is the total area of the jets, m.sub..theta. is the total tangential mass flow, and m.sub.a is the total axial mass flow.
For a conventional hub vane-swirler design without flow through the central hub, the swirl number equation (1) reduces to: ##EQU3## where R.sub.h is the radius of the central hub, .alpha. is the vane angle from the vertical axis, and U is uniform over the tube cross section. The conventional hub vane-swirler was designed for non-premixed combustion in which the central hub acts as a bluff-body and generates a torroidal recirculation zone. TRZs are formed only when there is a high degree of swirl in the flow field, that is, where S.gtoreq.approximately 0.6.
The swirl requirement of a weak-swirl burner (WSB) is different from that of other burners since the feed gas is premixed and flame stabilization is achieved through use of a divergent flow field instead of a TRZ. Due to the propagating nature of pre-mixed flames and the deceleration of the flow as it moves away from its source, the flame is able to dynamically stabilize itself at the position where the local mass flow rate balances the flame propagation speed. The weak-swirl stabilization mechanism does not apply to diffusion flames (not pre-mixed) because they do not propagate, but rather burn at the boundary where the air and fuel flows have diffused to the appropriate ratios for sustaining the combustion reaction.
The air-swirled weak-swirl burners disclosed in the '878 application are well suited for those applications where compressed air is readily available and currently used, such as in boilers and industrial furnaces. However, air injection may be impractical or uneconomical (due the necessary compressor and controls) for some high volume, low margin consumer products, for example, water heaters. Therefore, it would be highly desirable to replicate the benefits of the low NO.sub.x emission weak swirl burner of the '878 application with alternative designs, particularly if those designs were easily adaptable to a broad range of applications and were relatively simple and economical to scale, manufacture and operate.
Accordingly, there is a need for alternative designs for weak-swirl burners.